This application is the national phase under 35 U.S.C. xc2xa7371 of prior PCT International Application No., PCT/JP96/03443, which has an International filing date of Nov. 22, 1996, which designated the United States of America, the entire contents of which are hereby incorporated by reference.
The present invention relates to a channel estimation circuit for estimating the conditions of a channel through the use of a modeled channel prior to communication and to a modem having such a channel estimation circuit for use in a mobile communication system or mobile satellite communication system.
As a system for estimating the conditions of channels, there has been proposed a system that uses a six-beam antenna for each of transmission and reception and chooses the best route from a total of thirty-six channels.
An example of such conventional channel estimation systems there is illustrated in FIGS. 1 and 2, which are simplified versions of the figures presented in an article entitled xe2x80x9cA Technical Trend of High-speed Wireless Lansxe2x80x9d (Tsutsumi et al.), 6th Karuizawa Workshop on Circuit and System (1993). In FIG. 1, reference numeral 1 denotes a radio terminal called a user module (hereinafter referred to as UM) mounted on a table or the like, 2 a radio terminal which is called a control module (hereinafter referred to as CM) mounted on a ceiling of a room, for example, and connected with plural user modules UM1 by radio, and 3 schematically depicted walls of a room. Solid lines in FIG. 1 represent possible channels between CM2 and UM1. Terms xe2x80x9cGoodxe2x80x9d, xe2x80x9cBetterxe2x80x9d, and xe2x80x9cBadxe2x80x9d in cells shown in FIG. 2 imply that the channels associated with the cells have xe2x80x9cGoodxe2x80x9d, xe2x80x9cOrdinaryxe2x80x9d, and xe2x80x9cBadxe2x80x9d communication conditions, respectively. Void cells imply that the corresponding channels are not available for the current communication.
As shown in FIG. 1, CM2 and UM1 each have a six beam antenna, so that a total of thirty-six channels are available between CM2 and UM1. CM2 and UM1 are always aware of individual transmission qualities of the thirty-six channels in such a tabular form as depicted in FIG. 2, and may select the best one for use when the communication is actually performed.
The operation will now be described below.
It is supposed that CM2 and UM1 are arranged in a room surrounded by the walls 3 and that the channels between UM1 and CM2 at a given moment have such transmission qualities as shown in FIG. 1. At this time, only four channels are presented as indicated by the solid lines in FIG. 1. If the best channel is searched in the four channels, there is one way from the fifth antenna of CM2 to the sixth antenna of UM1. Thus, the channel from the fifth antenna of CM2 to the sixth antenna of UM1 will be selected when the communication is actually started.
Because the conventional channel estimation systems have such a configuration as described above, they use received signals to estimate the qualities of the channel and determine the channel for the current burst through utilization of a signal of the immediately preceding burst, whereby this gives rise to a problem that the system cannot follow quick changes on transmission quality.
In addition, such a conventional system needs continual monitoring of transmission qualities of the channel, and the transmission and reception it is performed between CM2 and UM1, also there is a problem that the efficiency in lines is lowered.
Further, the conventional channel estimation systems normally presume communication between fixed stations, and no consideration is given to a system which involves frequent channel switching by a radio terminal like UM1 during communication, with the radio terminal being moved.
The present invention is intended to overcome the above-mentioned problems, and has for its object to provide a channel estimation circuit capable of estimating the conditions of channels between a transmitter and a receiver with higher accuracy by obtaining models of the channel in geometrical form, then recognizing the position of the transmitting antenna through utilization of geometrical information based on the models, and estimating the path between the transmitter and the receiver.
It is another object to provide a channel estimation circuit that improves the transmission efficiency by geometrically estimating the path between the transmitter and the receiver.
It is a further object to provide a modem that permits high quality communication by following the path between the transmitter and the receiver through the use of geometrically modeled channels.
It is still a further object to provide a modem that permits fast channel switching by a terminal while moving during communication.
A channel estimation circuit according to the invention defined in claim 1 is one that first takes in geometrically ambient conditions by a geometrical data input device, then searches the input geometrical information for position information on the antenna of a distant station by a geometrical information recognition device, and geometrically formulates models of channels between radio terminals of the local station and the distant station by a modeling device through utilization of the searched-out position information on the antenna and the input geometrical information.
Therefore, it is possible to search for the antenna of the distant station with higher accuracy from its geometrically estimated position and estimate channel conditions with higher accuracy by using the channel models formulated from the input geometrical information, thereby eliminating the need for monitoring the transmission performance of every channel at all time and hence providing for increased line efficiency.
A channel estimation circuit according to the invention defined in claim 2 is one that is adapted to limit the range over which to search for the position of the antenna of the distant station through the use of information from an auxiliary geometrical information recognition device when the antenna position and surrounding buildings affecting the channels between the radio terminals of the local and distant stations are known at the time of geometrically formulating models of the channels.
Thus, it is possible to produce an effect of speeding up the search for the antenna of the distant station through the use of the information provided by the auxiliary geometrical information recognition device, in addition to the merit of the invention defined in claim 1.
A modem according to the invention defined in claim 3 is one that geometrically estimates the position of the antenna of the distant station by the use of the channel estimation circuit of claim 1, and enables the antenna of the local station to shift its directivity toward the antenna of the distant station.
Accordingly, the position of the antenna of the distant station can be estimated geometrically with accuracy and hence direct waves can be received with higher accuracy, therefore, it is possible to remove a multi-path component, thus producing an effect of achieving higher quality communication.
A modem according to the invention defined in claim 4 is one that formulates models of channels through the use of the channel estimation circuit of claim 1, estimates paths between radio terminals of the local and distant stations by a path estimating device based on the channel models, selects an optimal one of the estimated paths, and controls the antenna of the local station to shift its directivity toward the selected path.
Therefore, it is possible to produce the same effects as those obtainable with the invention of claim 3, and further, since the modem uses an optimal path based on the channel models for communication, high quality communication can be maintained even if the direct wave is blocked by an obstacle or even if sudden channel switching occurs.
A modem according to the invention defined in claim 5 is one that formulates models of channels through the use of the channel estimation circuit of claim 1, and eliminates or synthesizes multi-path components from received signals by a demodulator based on the channel models obtained with the channel estimation circuit.
Therefore, it is possible to lessen the influence of the multi-path component and hence bring about an effect of achieving high quality communication even if a non-directional antenna is used as the receiving antenna.
A modem according to the invention defined in claim 6 is one that formulates models of channels through the use of the channel estimation circuit of claim 1, always estimates by a frequency deviation calculating circuit, through utilization of the modeled channels, a frequency deviation due to a Doppler shift anticipated to occur following channel switching, and, when the channel switching actually occurs, provides the estimated frequency deviation as frequency information to the demodulator, which uses the frequency information to eliminate the frequency deviation due to the Doppler shift.
Therefore, it is possible to eliminate the frequency deviation due to the Doppler shift at the time of channel switching and hence produce an effect of achieving a quick initial pull-in operation.
A modem according to the invention defined in claim 7 is one that utilizes, in any one of the modems as defined in claims 3, 4, and 5, the frequency deviation calculating circuit mentioned in claim 6 to continually estimate a frequency deviation anticipated to occur due to the Doppler shift at the time of channel switching, and eliminates the frequency deviation due to the Doppler shift based on the estimated frequency deviation upon occurrence of channel switching.
Therefore, it is possible to produce the same effects as those obtainable with the inventions of f claims 3 through 5 and an effect of allowing a quick initial pull-in operation as is the case with the invention defined in claim 6.
A modem according to the invention defined in claim 8 is one that substitutes the channel estimation circuit of claim 7 with the channel estimation circuit of claim 2 adapted to limit the range over which to search for the antenna of the distant station through the use of information received from the auxiliary geometrical information recognition device when geometrically formulating models of channels between the radio terminals of the local and distant stations.
Therefore, it is possible to produce an effect of speeding up the search for the antenna of the distant station, in addition to the same effect as is obtainable with the invention of claim 7.
A modem according to the invention defined in claim 9 is one that formulates models of channels between the radio terminals of the local and distant stations through the use of the channel estimation circuit defined in claim 1, then estimates the path between the radio terminals of the both stations based on the channel models obtained with the channel estimation circuit and determines by a decision circuit whether an equalizer is needed or not according to the results of estimation, and provides a control signal to the equalizer to put it in or out of operation, depending on whether it is needed or not.
Therefore, it is possible to decide the ON/OFF control of the equalizer based on ambient conditions, permitting reduction of power consumption of the modem by stopping unnecessary operation of the equalizer.
A modem according to the invention defined in claim 10 is one that formulates channel models through the use of the channel estimation circuit of claim 1, then diagnoses by a channel diagnosing circuit the conditions of channels between the transmitter and the receiver based on the channels obtained with the channel estimation circuit, and determine the transmission rate in accordance with the results of diagnosis.
Therefore, it is possible to perform communication at the transmission rate corresponding to the results of diagnosis of the channels between the transmitter and the receiver, bringing about an effect of permitting communication at more optimum transmission rate.
A modem according to the invention defined in claim 11 is one that estimates the path between radio terminals of the local and distant stations by a path estimating device based on the channel models obtained with the channel estimation circuit of claim 1 as in the invention of claim 4, and controls transmission by a transmission control part through utilization of the path information from the path estimating device so as to lessen the influence of the multi-path components at the receiving side.
Therefore, since transmission is controlled at the transmitting side to lessen the influence of the multi-path components at the receiving side, no equalizer is needed and no multi-path components are received at the receiving side, whereby this permits high quality communication even if a non-directional antenna is used at the receiving side.
A modem according to the invention defined in claim 12 is one that formulates models of channels through the use of the channel estimation circuit of claim 1, always estimates by a frequency deviation calculating circuit through utilization of the channel models a frequency deviation due to the Doppler shift anticipated to occur at the time of channel switching, then provides the estimated frequency deviation as frequency information to a modulator upon occurrence of channel switching, and controls the transmission frequency by the modulator based on the frequency information so as to eliminate the frequency deviation due to the Doppler shift.
Therefore, since the frequency deviation due to Doppler shift can be eliminated in the modulator side at the time of channel switching, the frequency deviation need not be taken into account at the receiving side during channel switching, permitting faster initial pull-in operation.
A modem according to the invention defined in claim 13 is one that substitutes the channel estimation circuit of any one of claims 3 through 6 and 9 through 12 with the channel estimation circuit of claim 2 adapted to limit the range over which to search for the antenna of the distant station through the use of information received from the auxiliary geometrical information recognition device when geometrically formulating models of channels between the radio terminals of the local and distant stations.
Therefore, it is possible to produce an effect of speeding up the search for the antenna of the distant station, in addition to the same effect as is obtainable with the invention of any one of claims 3 through 6 and 9 through 12.